In a conventional technique in the field of a radar system, a “distributed aperture radar” is known in which the amplitude and phase of each of transmission and reception signals are controlled in a plurality of antennas (sub-arrays) distributed and fixedly arranged in predetermined positions, to perform beam synthesis.
As the measurement technique of a relative position of each of the plurality antennas (sub-arrays) distributed in the distributed aperture radar, (1) a measuring method by GPS, (2) a measuring method of tracking a pseudo target such as a metal sphere by each of the plurality of antennas with radar wave, and (3) a measuring method by installing a laser ranging apparatus on each antenna and tracking a laser beam. The details of the methods will be described below.
(1) the Measuring Method by GPS
The measurement by use of GPS has a measurement precision of 10 to 100 m in a single point positioning (single point instant measurement). Also, when interference position measurement such as static measurement (measurement by the GPS receivers at a known point and a measurement point for a time period of 30 minutes to 1 hour) is used, the measurement precision is an order of a few centimeters. Depending on the arrangement of the GPS satellites, the measurement precision is degraded.
(2) the Measuring Method by Tracking a Pseudo Target Antenna Such as a Metal Sphere by Each Antenna with Radar Wave
When the pseudo target such as the metal sphere is tracked with the radar wave to measure a relative position, a measured relative position has a great variation because enough angular measurement precision cannot be obtained in the antennas (sub-arrays) distributed in a small area. In a single antenna, the antenna aperture is small so that a beam width spreads. As a result, the enough angular measurement precision cannot be attained. For this reason, in order to improve the angular measurement precision, the antennas are distributed in a large area to configure a distributed aperture antenna (large distributed aperture antenna).
Also, in the measurement using the GPS and the metal sphere, an approximate position of each of the antennas (sub-arrays) can be only measured in an order of the wavelength of the radar wave. Therefore, the transmission and reception of the radar wave in each antenna (sub-array) cannot be precisely controlled based on the relative position of the antenna (sub-array) even in the distributed aperture radar. That is, a correct beam cannot be formed. For example, when the wavelength of the radar wave used in the distributed aperture radar is in a range of several cm to several mm, the beam formation is influenced if the measurement precision is about several cm. That is, a limit is in the measurement precision in the position/direction measurement using the GPS and the metal sphere.
(3) the Measuring Method in which a Laser Ranging Apparatus is Installed on Each Antenna to Track with a Laser Beam
When a distance measurement is carried out by using a laser beam, enough distance/direction measurement precision can be attained because the wavelength of the laser beam is sufficiently small, compared with that of the radio wave. In this case, however, if the laser ranging apparatus is installed on the each of the antennas (sub-arrays) to configure an alignment apparatus, it is disadvantageous in view of cost because the laser ranging apparatus is very expensive.
Patent Literature 1 (Japanese Patent No. 2,500,377) discloses a technique that a warp of a main mirror of a parabolic antenna for a satellite (a variation quantity from an ideal parabolic curved surface) is measured from a distant place. In the technique, light beams (4 incident beams, e.g. 4 laser beams) are irradiated from the earth to the main mirror of the satellite. Also, the light beams are reflected by warp measurement areas (e.g. 3 small concave mirrors) provided to be distributed on the antenna main mirror surface. Also, the reflection light beams from the warp measurement areas are collected and detected at the same time by an imaging sensor arranged on a beam collection position, so as to obtain a spot image. A variation from the ideal antenna main mirror surface is measured based on a difference between a reference spot image obtained from the ideal antenna main mirror surface and the spot image obtained at the time of measurement of mirror surface warp. However, in Patent Literature 1, the plurality of warp measurement areas (e.g. 3 small concave mirrors) have the function to merely reflect the incident beams.